I have Sleep Paralysis (that disorder where your entire body locks up, unable to move, and barely breathe, and you're caught in a thin fissure between deep R.E.M. sleep and being awake), and I once had visual hallucinations, though I couldn't remember them at all, thirty seconds after it happened. The strange thing is, the next time I had an attack, they were AUDIO hallucinations, and I quickly realized that they weren't hallucinations, but were actually audio MEMORIES of random stuff I had heard in the past year or so. It would start with just one audio memory, then rapidly build up into a cacophonous collection of catastrophic craziness, where the audio memories were absolutely DEAFENING inside my own head. And of course, being unable to move in that state, I can't shake myself from the horror, or at least ineffectually cover my ears to provide some sort of comfort and false sense of protection against the noise.
To this day I wonder if my visual hallucinations were also memories, and that maybe when the brain is storing short-term memories into long-term memories, it solidifies audio and visual separately.